Current electronic parallel computers are capable of processing data at rates well into the GFLOP range. These peak rates can be achieved only when the data involved reside in the main memory of the system and can be transferred to the processing units at high speeds.
This is typically the case with numerically intensive problems such as scientific simulations and floating point calculations. A different class of problems, known collectively as non-numerical processing, includes data and knowledge base management, logic inferencing, image processing, machine vision, computer-generated holographic storage, and document retrieval. These applications are input/output intensive and rely on the computer's ability to process a vast amount of data which cannot fit in main memory but must be retrieved from secondary storage.
Therefore, the performance of the secondary memory system becomes critical and is usually determined by two factors: storage capacity (in Mbytes) and data transfer rates (in Mbytes per second). Both of these quantities must be as high as possible. In order for storage devices to keep up with the constant increase in database volumes, they must allow access to Terabytes of data in a relatively short time.
With existing technology the most inexpensive means of storing information is magnetic storage. Magnetic storage allows the storage of large amounts of data but has a number of limitations. Magnetic or electric fields can erase or corrupt the data, the storage medium can be tampered with, and the read/write process limits the data transfer rate achievable.
Optical storage media based on holographic data storage have greatly increased the amount of data which can be stored as well as improving security and data access time. The most common application of this technology is in audio or video compact disks.
The audio CD was introduced jointly by Philips and Sony in 1982. It stores digital bits as pits (or the absence of pits) impressed in its reflective surface along concentric tracks. Transparent plastic protects the surface, which is scanned by the beam of a solid-state laser having a 780-nm wavelength. The audio CD stores 640-680 MB of information, or about 74 minutes of music, assuming standard sampling rate, frequency, and encoding.
Two computing proposals for high-density disks have been announced: the Sony/Philips MultiMedia CD (MMCD) and the Toshiba/Time Warner Super-Density (SD) disk. As currently proposed, the latter is a two-layer disk that can hold 3.7 GB on a single layer, for a total capacity of 7.4 GB. The proposed SD disk stores 5 GB on each side, for a total of 10 GB.
Storage capacities on compact disc systems are large and thus very useful but limited by the mechanical parts and, to some extent, by the cost. Furthermore, the majority of known holographic data storage systems are read only. Such systems include CD ROM and WORM (write once read many).
Among various other storage systems, three-dimensional (3-D) optical memories, such as volume holograms and two-photon memories, appear very attractive. Holographic storage offers large digital storage capacity, fast data transfer rates, and short access times. Current storage technologies are limited in that they do not simultaneously provide each of these three features.
Erasable devices based on thermoplastic have been developed quite recently. One such device is that described in EP 256554 in the name of Teijin KK This patent describes a thermoplastic substrate for use in optical memory cards. The patent details a laminated card structure having defined lamination thicknesses to improve image contrast.
A random access optical memory device has been described by Uban and Urban in U.S. Pat. No. 5,311,474. The optical storage medium is divided into a matrix of storage fields which are accessed in parallel by using a beam multiplexer to simultaneous address each storage field.
Other known digital holographic storage techniques include storage of microfiche onto Mylar substrates using a holographic image storing technique whereby the microfiche is stored onto a master plate and the master is converted to an embossing master for stamping out onto Mylar or polyester substrates. Once the image or code is placed onto the master plate it is not changed in any way.
The inventor is not aware of any compact, high data volume, holographic storage device that is `write many read many` (eg. erasable and rewritable).
Presently, processor speed far exceeds the ability of conventional rotating disk devices to import or to export the data needed for manipulations. As a result most computers are I/O bound. This I/O gap is widening at a steady rate as digital processor technology continually advances. Holographic data storage devices have the potential to overcome this gap by offering much faster data access time and higher data band-width than for conventional rotating disk devices having similar capacity in the gigabyte range. In addition to computer applications holographic data storage systems may be useful for optical interconnects, multimedia telecommunications and associative readout in optical processors to mention just a few examples. In particular, optical telecommunication networks offer data transfer rates in the gigabit per second range commensurate with potential holographic data storage system capabilities.
The usual holographic recording process involves the interference of two coherent, parallel polarisation light beams on an appropriate photosensitive material (photopolymerisable/photocrosslinkable). It is accomplished by combining an image-bearing light beam and a reference beam in a recording medium. The variation in intensity in the resulting interference pattern causes the complex index of refraction to be modulated throughout the volume of the medium.
In a bulk photorefractive medium, such as LiNbO.sub.3, charges are excited from impurity centres in the presence of light and subsequently trapped. The resulting space-charge causes modulation in the index of refraction through the electro-optic effect. When the medium is exposed to a reference beam identical to one used in recording, the light will diffract in such a way as to reproduce the original image-bearing wavefront.
In holographic data storage, data are converted to an optical signal by use of an SLM (spatial light modulator). A hologram corresponding to the image (one data "page") on the SLM is then recorded in a photorefractive crystal or other suitable volume holographic recording medium.
There are two established methods of multiplexing the holograms within the recording medium. The first is spatial multiplexing, in which the data are stored in spatially adjacent but separate areas of the medium. This is often done by forming two-dimensional Fourier-transform hologram arrays and is most useful for thin surface-storage media. The second is angular multiplexing, in which holograms are completely superimposed within a common volume. This technique is most useful for thick volume-storage media such as photorefractive crystals because of their highly angularly selective behaviour arising from their thickness.
Optical heads have been described in the prior art for use with known optical data storage systems. The most common is the optical reader user in compact disc readers. These heads comprise a laser diode that illuminates the disc and a detector that records laser light reflected from the disk A signal processing algorithm derives digital information from the reflected signals which is transformed to the desired data.
Other optical heads incorporating specific features are also known. Reference may be had to U.S. Pat. No. 5,148,421 assigned to Matsushita Electric Industrial Co Ltd which describes an optical head for rewritable discs. The head is designed for use with either single-sided or double-sided discs and incorporates correcting optics to account for aberrations. An actuator means positions the correcting optics. The mechanical components and complexity of the optical train limits the application of the device.
Reference may also be had to German Patent number DE3717605 assigned to Olympus Optical Co Ltd. This patent describes an optical read-write head for cards that uses a laser for writing and an LED for reading. A multi-segment detector is used to provide accurate focal length control and thereby increase recording density. The head is not able to perform a complete read/write/erase cycle.
It is clear from the above discussion that a number of issues need to be addressed in the design of a holographic data storage system. Crosstalk between superimposed images reduces the useful data capacity and is a major contributor to the overall system noise. As a result there is a tradeoff between data capacity and the achievable signal-to-noise ratio or bit error rate. It is preferable that the resolution of the storage process be limited by the optics or by the wavelength of light used rather than by the recording medium chosen.
A viable holographic storage system depends on suitable key component technologies, particularly the optical head and the optical storage medium.